The influence of varying contents of ferric and ferrous iron on the mineral assemblages developed in regionally metamorphosed pelitic gneisses from Glen Clova, Angus, is considered on the basis of petrographic, chemical, and X-ray data. The rocks described are essentially quartz-oligoclase-muscovite-biotite-garnet-kyanite-oxide minerals gneisses, with oxidation ratios varying from 6 to 75; rocks of widely varying oxidation ratio may be found intimately interbedded.